High voltage power semiconductor devices are used in many power conversion and control applications. Such devices include diodes, bipolar transistors, insulated gate bipolar transistors (IGBTs), thyristors, and metal oxide field effect transistors (MOSFETs). At first glance, it would appear that the physics of semiconductor high voltage power devices is identical to their low voltage power counterparts. However, although some features of device operation are similar, many aspects of device physics take on a different sense of importance under conditions of high voltage operation.
In general, high voltage semiconductor devices (i.e., devices capable of withstanding breakdown voltages in excess of 200 volts or more) must be capable of supporting high blocking voltages when in an OFF state, and capable of conducting at high current levels with minimum power dissipation (i.e., low on-resistance) when in an ON state. However, a high blocking voltage and a low on-resistance present two conflicting design parameters that challenge designers and fabricators of high voltage devices.
The reverse breakdown voltage of a diode junction device is determined by, among other things, the resistivity of the p- and n-regions, and by doping profiles. High voltage devices require manufacturers to use high resistivity starting materials and/or regions. By way of example, resistivity values greater than 1 to 3 ohm-cm or more are common for high voltage devices compared to resistivity values of 0.15 to 0.20 ohm-cm for typical integrated circuit devices. The higher resistivity values coupled with evolving design constraints present challenges that make it difficult to produce high voltage power devices that are stable and reliable over time. Such design constraints include smaller device dimensions, multiple levels of metallization, and integration with other sensitive devices (e.g., integrated circuit devices).
Additionally, present methods and structures used for lower voltage devices have been found to be unreliable when used for higher voltage devices. For example, such methods and structures result in devices having breakdown voltage degradation over time, unwanted field inversion effects, and parasitic leakage effects.
Accordingly, improved power semiconductor device structures and methods of their manufacture are needed to address the above mentioned issues as well as others.